Retrofitting Existing OWSJ for RTU

[CANeng11]

I am currently waiting on the original fabricator of a building to send me information on the installed OWSJ in a building. The current occupant is looking to add a 700 lb RTU between two joists. Additional snow drift surcharge can be ignored by my local code for a unit this size. Whats the best document on reinforcing these joists?

 

[skeletron]

I would search out the documentation from Structural Joist Institute first.
For example:

https://steeljoist.org/wp-content/uploads/SJI_JulyWebinar_Retrofitting_FINAL_071219.pdf

I would also search out the historical joist design if you can find the original manufacturer.

In all cases, you may need to model the joist size based on field measurements. This is approximate and you have to be aware of the limitations of your model, but it can at least help fence in the solution before you commit.

 

[CANeng11]

If I have the joist series number (22k4), is there a document that shows the sizes of all the members?

 

[skeletron]

Google is your friend. Last time I had to do this I needed to track down an old Stelco model (different joist manufacturer). Be prepared for any old document to not give you much except for vague section properties (in my experience) or allowable stress.

 

[phamENG]

CANeng11 - to answer your question, no. The joist designator is tied to a minimum strength and stiffness requirement. In other words, all SJI member fabricators have to produce a 22K4 that can support the loads listed in the SJI tables at the given spans. How they get there is up to them, and is how they develop their economy and compete with one another.

That said, somebody did put a document together that can be useful for estimating the chord designator for a K series joist. It's attached here. THIS IS NOT A GUARANTEE. These sizes are approximate and is a useful tool for evaluating existing buildings when you don't have drawings, and can also give you some insight into the size material that is LIKELY used.

If you know the designator and the fabricator, tracking down their load tables shouldn't be too bad. From that you can build the typical shear and moment diagrams for a uniformly loaded joist. Compare your new shear and moment diagram and make sure you don't cross it.

 

[KootK]

1) I have a copy of SJI's 75 joist manual doc which can be useful. It's a US thing though and, like phamENG said, it doesn't tell you what the members are going to be. That said, having the spec and the load tables can be very handy for:

a) figuring out if you need to reinforce.

b) estimating stiffness.

c) estimating minimum shear capacity which equates roughly to estimating joint weld capacity and seat capacity.

2) Again like phamENG said, your ideal situation here would be to be able to lock in the age of the building and the manufacturer of the joist such that you might hit up their decedents for some more detailed information. If you know that this is a 22k4, does that mean that there's an ID tag on the joist seat that might ID the manufacturer? Or did you just get that from the structural drawings? Unfortunately, in AB we've had a zillion different kinds of joists manufactured by numerous suppliers who have since gone out of business or consolidated. As I understand it, the local suppliers don't retain their old data beyond about 1980.

3) You might do well to post some pics of your joists here. Somebody like BAretired who has a very extensive history in AB may well be able to provide some critical insight.

 

[milkshakelake]

SJI (Steel Joist Institute) can identify the joist for you for free and possibly give a load table. You'd need field measurements and a sketch.

https://steeljoist.org/joist-investigation-form/

The load table would only have uniform loading, so you'd have to run some numbers and make an engineering judgment.

 

[msquared48]

If you put the RTU next to a column, the joists become a minor issue.

Mike McCann, PE, SE (WA, HI)